Image sensors with enhanced wide-angle performance

ABSTRACT

Imaging apparatus (2000, 2100, 2200) includes a photosensitive medium (2004, 2204) and an array of pixel circuits (302), which are arranged in a regular grid on a semiconductor substrate (2002) and define respective pixels (2006, 2106) of the apparatus. Pixel electrodes (2012, 2112, 2212) are connected respectively to the pixel circuits in the array and coupled to read out photocharge from respective areas of the photosensitive medium to the pixel circuits. The pixel electrodes in a peripheral region of the array are spatially offset, relative to the regular grid, in respective directions away from a center of the array.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of the following U.S. provisionalpatent applications, whose disclosures are incorporated herein byreference:

-   -   U.S. Provisional Patent Application 62/410,792, filed Oct. 20,        2016;    -   U.S. Provisional Patent Application 62/410,793, filed Oct. 20,        2016;    -   U.S. Provisional Patent Application 62/410,797, filed Oct. 20,        2016;    -   U.S. Provisional Patent Application 62/411,497, filed Oct. 21,        2016;    -   U.S. Provisional Patent Application 62/411,517, filed Oct. 21,        2016;    -   U.S. Provisional Patent Application 62/411,519, filed Oct. 21,        2016; and    -   U.S. Provisional Patent Application 62/411,522, filed Oct. 21,        2016.

FIELD OF THE INVENTION

The present invention relates generally to image sensing devices, andparticularly to circuits and methods for enhancing the performance offilm-based image sensors.

BACKGROUND

In film-based image sensors, a silicon-based switching array is overlaidwith a photosensitive film such as a film containing a dispersion ofquantum dots (referred to herein as a “quantum film”). The switchingarray, which can be similar to those used in complementary metal-oxidesandwich (CMOS) image sensors that are known in the art, is coupled bysuitable electrodes to the film in order to read out the photochargethat accumulates in each pixel of the film due to incident light.

SUMMARY

Embodiments of the present invention that are described hereinbelowprovide enhanced image sensor designs and methods for operation of imagesensors with enhanced performance.

There is therefore provided, in accordance with an embodiment of theinvention, imaging apparatus, including a photosensitive medium and anarray of pixel circuits, which are arranged in a regular grid on asemiconductor substrate and define respective pixels of the apparatus.Pixel electrodes are connected respectively to the pixel circuits in thearray and coupled to read out photocharge from respective areas of thephotosensitive medium to the pixel circuits. The pixel electrodes in aperipheral region of the array are spatially offset, relative to theregular grid, in respective directions away from a center of the array.

In a disclosed embodiment, the photosensitive medium includes a quantumfilm.

In some embodiments, the pixel electrodes in the peripheral region ofthe array are enlarged in the respective directions relative to thepixel electrodes in the center of the array.

Typically, the apparatus includes objective optics, which are configuredto form an image of an object on the photosensitive medium, and thepixel electrodes in the peripheral region are spatially offset by adisplacement determined by a chief ray angle of the objective optics.

In a disclosed embodiment, the apparatus includes microlenses formedover the photosensitive medium, wherein the microlenses associated withthe pixels in the peripheral region of the array are spatially offset,relative to the regular grid, in respective directions toward the centerof the array.

There is also provided, in accordance with an embodiment of theinvention, a method for producing an image sensor. The method includesforming an array of pixel circuits in a regular grid on a semiconductorsubstrate, thereby defining respective pixels of the image sensor. Pixelelectrodes are connected respectively to the pixel circuits in thearray, wherein the pixel electrodes in a peripheral region of the arrayare spatially offset, relative to the regular grid, in respectivedirections away from a center of the array. The pixel electrodes arecoupled to read out photocharge from respective areas of thephotosensitive medium to the pixel circuits.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a camera module, which is operativein accordance with an embodiment of the invention;

FIG. 2 is a schematic top view of an example image sensor, in accordancewith an embodiment of the invention;

FIGS. 3A-3C are schematic sectional side views of example pixels ofimage sensors in accordance with embodiments of the invention;

FIGS. 4A and 4B are electrical circuit diagrams that schematicallyillustrate pixel circuits in an image sensor, in accordance withembodiments of the invention;

FIG. 5 is a schematic top view of an image sensor with selective readoutcapability, in accordance with an embodiment of the invention;

FIGS. 6 and 7 are timing diagrams, which schematically illustrate timingsignals applied by row logic in an image sensor, in accordance withembodiments of the invention;

FIG. 8 is a table showing a flow of pointers used in controlling rowlogic in an image sensor, in accordance with an embodiment of theinvention;

FIG. 9 is a schematic top view of a color image sensing array,illustrating a method for correction of crosstalk in accordance with anembodiment of the invention;

FIGS. 10 and 11 are plots of voltage at the sense nodes of two pixels inan image sensor, illustrating the use and effect of a reset lock signalin accordance with embodiments of the invention;

FIG. 12 is a schematic sectional view of a part of an image sensor,showing an optically black area formed in accordance with an embodimentof the invention;

FIG. 13 is a schematic, sectional illustration of a dual image sensorassembly, in accordance with an embodiment of the invention;

FIG. 14 is a schematic sectional illustration of an imaging module withdual sensing areas, in accordance with another embodiment of theinvention;

FIG. 15 is a schematic top view of an image sensor chip with dual imagesensing areas, in accordance with a further embodiment of the invention;

FIG. 16 is a schematic sectional view of a part of an image sensor,illustrating an example implementation of autofocus pixels, inaccordance with an embodiment of the invention;

FIG. 17 is a schematic top view of a part of an image sensor includingautofocus pixels, in accordance with another embodiment of theinvention;

FIG. 18 is a schematic top view of a part of an image sensor includingautofocus pixels, in accordance with yet another embodiment of theinvention

FIG. 19 is a schematic sectional view of a part of an image sensor,illustrating an alternative implementation of autofocus pixels, inaccordance with an embodiment of the invention;

FIG. 20A is a schematic sectional view of an image sensor with enhancedacceptance of high chief ray angles, in accordance with an embodiment ofthe invention;

FIG. 20B is a schematic top view of the image sensor of FIG. 20A;

FIG. 21 is a schematic top view of a part of an image sensor withenhanced acceptance of high chief ray angles, in accordance with anotherembodiment of the invention; and

FIG. 22 is a schematic sectional view of a part of an image sensor withenhanced acceptance of high chief ray angles, in accordance with yetanother embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS System Overview

The image sensors described herein may be used within any suitableimaging device, such as a camera, spectrometer, light sensor, or thelike. FIG. 1 shows one example of a camera module 100 that may utilizean image sensor 102, which may be configured in any manner as describedbelow. The camera module 100 may comprise a lens system 104, which maydirect and focus incoming light onto image sensor 102. While depicted inFIG. 1 as a single element, it should be appreciated that lens system104 may actually include a plurality of lens elements, some or all ofwhich may be fixed relative to each other (e.g., via a lens barrel orthe like). The camera module 102 may optionally be configured to movethe lens system 104 and/or the image sensor 102 to perform autofocusand/or optical image stabilization.

The camera module may further comprise one or more optional filters,such as a filter 106, which may be placed along the optical path. Filter106 may reflect or otherwise block certain wavelengths of light, and maysubstantially prevent, based on the effectiveness of the filter, thesewavelengths of light from reaching the image sensor 102. As an example,when an image sensor is configured to measure visible light, filter 106may comprise an infrared cutoff filter. While shown in FIG. 1 as beingpositioned between image sensor 102 and lens system 104, filter 106 maybe positioned to cover lens system 104 (relative to incoming light) ormay be positioned between lenses of lens system 104.

FIG. 2 shows a top view of an exemplary image sensor 200 as describedherein. Image sensor 200 may comprise an imaging area comprising a pixelarray 202, which may include a first plurality of pixels 212 that may beused to convert incident light into electrical signals. In someinstances, pixel array 202 may comprise an obscured region 210 includingat least one pixel (e.g., a second plurality of pixels) that is obscuredrelative to incoming light (e.g., covered by a light-blocking layer).Electrical signals may still be read out from some or all of thesepixels, but since there is ideally no light reaching these pixels, thecurrent measured from these pixels may represent the dark currentassociated with one or more components of the image sensor. Image sensor200 (or associated processing circuitry) may compensate for the darkcurrent levels during image capture and/or processing.

Image sensor 200 may further comprise row circuitry 204 and columncircuitry 206, which collectively may be used to convey various signals(e.g., bias voltages, reset signals) to individual pixels as well as toread out signals from individual pixels. For example, row circuitry 204may be configured to simultaneously control multiple pixels in a givenrow, while column circuitry 206 may convey pixel electrical signals toother circuitry for processing. Accordingly, image sensor 200 maycomprise control circuitry 208, which may control the row circuitry 204and column circuitry 206, as well as performing input/output operations(e.g., parallel or serial IO operations) for image sensor 200. Thecontrol circuitry may include a combination of analog circuits (e.g.,circuits to provide bias and reference levels) and digital circuits(e.g., image enhancement circuitry, line buffers to temporarily storelines of pixel values, register banks that control global deviceoperation and/or frame format).

FIG. 3A is a schematic cross-sectional side view of an example pixel300, which may be used in the image sensors described herein (such aspixel array 202 of image sensor 200 described above in relation to FIG.2). Pixel 300 may comprise a pixel circuitry layer 302 and aphotosensitive material layer 304 overlying pixel circuitry layer 302.Pixel circuitry layer 302 includes pixel circuitry for applying controlsignals to and reading out charge collected from photosensitive materiallayer 304.

Photosensitive material layer 304 may be configured to absorb photonsand generate one or more electron-hole pairs in response to photonabsorption. In some instances, photosensitive material layer 304 mayinclude one or more films formed from quantum dots, such as thosedescribed in U.S. Pat. No. 7,923,801, which is incorporated herein byreference in its entirety. The materials of photosensitive materiallayer 304 may be tuned to change the absorption profile ofphotosensitive material layer 304, whereby the image sensor may beconfigured to absorb light of certain wavelengths (or range ofwavelengths) as desired. It should be appreciated that while discussedand typically shown as a single layer, photosensitive material layer 304may be made from a plurality of sub-layers. For example, thephotosensitive material layer may comprise a plurality of distinctsub-layers of different photosensitive material layers.

Additionally or alternatively, photosensitive material layer 304 mayinclude one or more sub-layers that perform additional functions, suchas providing chemical stability, adhesion or other interface propertiesbetween photosensitive material layer 304 and pixel circuitry layer 302,or for facilitate charge transfer across the photosensitive materiallayer 304. It should be appreciated that sub-layers of photosensitivematerial layer 304 may optionally be patterned such that differentportions of the pixel circuitry may interface with different materialsof the photosensitive material layer 304. For the purposes of discussionin this application, photosensitive material layer 304 will be discussedas a single layer, although it should be appreciated that a single layeror a plurality of different sub-layers may be selected based on thedesired makeup and performance of the image sensor.

To the extent that the image sensors described here comprises aplurality of pixels, in some instances a portion of photosensitivematerial layer 304 may laterally span multiple pixels of the imagesensor. Additionally or alternatively, photosensitive material layer 304may be patterned such that different segments of photosensitive materiallayer 304 may overlie different pixels (such as an embodiment in whicheach pixel has its own individual segment of photosensitive materiallayer 304). As mentioned above, photosensitive material layer 304 may bein a different plane from pixel circuitry layer 302, such as above orbelow the readout circuitry relative to light incident thereon. That is,the light may contact photosensitive material layer 304 without passingthrough a plane (generally parallel to a surface of the photosensitivematerial layer) in which the readout circuitry resides.

In some instances, it may be desirable for photosensitive material layer304 to comprise one or more direct bandgap semiconductor materials whilepixel circuitry layer 302 comprises an indirect bandgap semiconductor.Examples of direct bandgap materials include indium arsenide and galliumarsenide, among others. The bandgap of a material is direct if amomentum of holes and electrons in a conduction band is the same as amomentum of holes and electrons in a valence band. Otherwise, thebandgap is an indirect bandgap. In embodiments in which pixel circuitrylayer 302 includes an indirect bandgap semiconductor and photosensitivematerial layer 304 includes a direct bandgap semiconductor,photosensitive material layer 304 may promote light absorption and/orreduce pixel-to-pixel cross-talk, while pixel circuitry layer 302 mayfacilitate storage of charge while reducing residual charge trapping.

The pixel circuitry in pixel circuitry layer 302 typically comprises atleast two electrodes for applying a bias to at least a portion ofphotosensitive material layer 304. In some instances, these electrodesmay comprise laterally-spaced electrodes on a common side of thephotosensitive material layer 304. In other variations, two electrodesare on opposite sides of the photosensitive material layer 304. In thesevariations, pixel 300 may comprise a top electrode 306 positioned overphotosensitive material layer 304. In embodiments that include a topelectrode, the image sensor is positioned within an imaging device suchthat oncoming light passes through top electrode 306 before reachingphotosensitive material layer 304. Accordingly, it may be desirable fortop electrode 306 to be formed from a conductive material that istransparent to the wavelengths of light that the image sensor isconfigured to detect. For example, top electrode 306 may comprise atransparent conductive oxide. In some instances, electrode 306 may spanmultiple pixels of an image sensor. Additionally or alternatively,electrode 306 optionally may be patterned into individual electrodessuch that different pixels have different top electrodes. For example,there may be a single top electrode that addresses every pixel of theimage sensor, one top electrode per pixel, or a plurality of topelectrodes wherein at least one top electrode address multiple pixels.

In some instances pixel 300 may further comprise one or more filters 308overlaying the photosensitive material layer 304. In some instances, oneor more filters may be common to the pixel array, which may beequivalent to moving filter 106 of FIG. 1 into image sensor 102.Additionally or alternatively, one or more of filters 308 may be used toprovide different filtering between different pixels or pixel regions ofthe pixel array. For example, filter 308 may be part of a color filterarray, such as a Bayer filter, CMY filter, or the like.

Additionally, in some variations the pixel 300 may comprise a microlensoverlying at least a portion of the pixel. The microlens may aid infocusing light onto photosensitive material layer 304.

FIG. 3B is a schematic cross-sectional side view of a variation of apixel 301, which shows a portion of pixel circuitry layer 302 in largedetail. Common components to those described in FIG. 3A are labeled withthe same numbers as in FIG. 3A. Pixel circuitry layer 302 can include asemiconductor substrate layer 312 and/or one or more metal layers(collectively referred to herein as metal stack 314) which collectivelyperform biasing, readout, and resetting operations of the image sensor.Semiconductor substrate layer 312 may include a semiconductor materialor combination of materials, such as silicon, germanium, indium,arsenic, aluminum, boron, gallium, nitrogen, phosphorus, doped versionsthereof. In one or more embodiments, semiconductor layer 312 includes anindirect-bandgap semiconductor (e.g., silicon, germanium,aluminum-antimonide, or the like). In instances in which the pixelcircuitry comprises a metal stack 314, the metal layers may be patternedto form contacts, vias, or other conductive pathways which may beinsulated by a dielectric such as SiO2. It should be appreciated thatmetal stack 314 and the associated interconnect circuitry may be formedusing traditional complementary metal-oxide semiconductor (CMOS)processes.

As shown in FIG. 3B, metal stack 314 may comprise a pixel electrode 316,which along with a second electrode (e.g., a laterally-spaced electrodeor a top electrode 306) may provide a bias to the photosensitive layerduring one or more operations of the image sensor. The metal layers mayfurther form a via between metal stack 314 and semiconductor substratelayer 312 to provide a connection therebetween.

To facilitate the collection and transfer of charge within the pixel,one or more transistors, diodes, and photodiodes may formed in or on asemiconductor substrate layer 312, for example, and are suitablyconnected with portions of metal stack 314 to create a light-sensitivepixel and a circuit for collecting and reading out charge from thepixel. Pixel circuitry layer 302 may facilitate maintaining storedcharges, such as those collected from the photosensitive layer. Forexample, semiconductor substrate layer 312 may comprise a sense node318, which may be used to temporarily store charges collected from thephotosensitive layer. Metal stack 314 may comprise first interconnectcircuitry that provides a path from pixel electrode 316 to sense node318. While metal stack 314 is shown in FIG. 3B as providing a directpathway between pixel electrode 316 and sense node 318 withoutintervening circuitry, it should be appreciated that in other instances(such as in circuitry described below with reference to FIG. 4B), one ormore intervening circuit elements may be positioned between the pixelelectrode 316 and the sense node 318.

FIG. 3C shows another variation of a pixel 303, which is similar topixel 301 of FIG. 3B (with common components from FIG. 3B labeled withthe same numbers), except that pixel 303 comprises a plurality ofseparate photosensitive layers, which may each provide electricalsignals. As shown in FIG. 3C, pixel 303 may comprise a firstphotosensitive layer 304 a and a second photosensitive layer 304 boverlying first photosensitive layer 304 a. An insulating layer 324 mayseparate first photosensitive layer 304 a from second photosensitivelayer 304 b, such that each photosensitive layer may be independentlybiased. Accordingly, pixel 303 may comprise a plurality of electrodes toprovide a respective bias to each of first photosensitive layer 304 aand second photosensitive layer 304 b. For example, in the variationshown in FIG. 3C, pixel 303 may comprise a first electrode 316 connectedto first photosensitive layer 304 a, a second electrode 322 connected tosecond photosensitive layer 304 b, and one or more common electrodes(shown as two electrodes 306 a and 306 b, although these electrodes maybe combined into a single electrode) connected to both the first andsecond photosensitive layers around at least a portion of the peripheryof pixel 303.

To reach second photosensitive layer 304 b, at least a portion of secondelectrode 322 may pass through a portion of first photosensitive layer304 a and insulating layer 324. This portion of second electrode 322 maybe insulated to insulate the second electrode from first photosensitivelayer 304 a. A first bias may be applied to first photosensitive layer304 a via first electrode 316 and the common electrodes, and a secondbias may be applied to second photosensitive layer 304 b via secondelectrode 322 and the common electrodes. While shown in FIG. 3C assharing one or more common electrodes, the first and secondphotosensitive layers need not share any electrodes. For example, thefirst and second photosensitive layers (and corresponding electrodes)may be configured in any suitable fashion, such as those described inU.S. Patent Application Publication 2016/0155882 the contents of whichare incorporated herein by reference in their entirety.

Each photosensitive layer may be connected to the pixel circuitry insuch a way that the photosensitive layers may be independently biased,read out, and/or reset. Having different photosensitive layers may allowthe pixel to independently read out different wavelengths (or wavelengthbands) and/or read out information with different levels of sensitivity.For example, first photosensitive layer 304 a may be connected to afirst sense node 318 while second photosensitive layer 304 b may beconnected to a second sense node 320, which in some instances may beseparately read out to provide separate electrical signalsrepresentative of the light collected by the first and secondphotosensitive layers respectively.

FIGS. 4A and 4B show example pixel circuitry which may be used to bias,read out, and reset individual pixels. While FIG. 4A shows a threetransistor (3T) embodiment and FIG. 4B shows a four transistor (4T)embodiment, it should be appreciated that these are just exemplarycircuits and any suitable pixel circuitry can be used to perform theseoperations. For example, suitable pixel circuitry embodiments aredescribed in US Patent Application Publications 2017/0264836,2017/0208273, and 2016/0037114, the contents of each of which areincorporated herein by reference in their entirety.

Turning to FIG. 4A, the pixel circuitry may be configured to apply afirst bias potential V_(BiasT), which may be applied to photosensitivelayer 400 (e.g., via a first electrode such as a top electrode asdiscussed above). Photosensitive layer 400 may also be connected to asense node 402 (e.g., via a pixel electrode such as discussed above).Sense node 402 may be connected to a second bias potential V_(BiasB) viaa reset switch 404 (which is controlled by a reset signal RESET). Resetswitch 404 may be used to reset sense node 402 at various points duringoperation of the image sensor. The pixel circuit of FIG. 4B is identicalto that of FIG. 4A, except that in FIG. 4B the pixel circuit includes atransfer switch 410 positioned between photosensitive layer 400 and thesense node. The transfer switch may be used to facilitate transfer ofcharge between photosensitive layer 400 and the pixel output.

Sense node 402 may further be connected to an input of a source followerswitch 406, which may be used to measure changes in sense node 402.Source follower switch 406 may have its drain connected to a voltagesource V_(SUPPLY) and its source connected to a common node with thedrain of a select switch 408 (controlled by a select signal SELECT). Thesource of select switch 408 is in turn connected to an output busCOLUMN. When select switch 408 is turned on, changes in sense node 402detected by follower switch 406 will be passed via select switch 408 tothe bus for further processing.

The image sensors described here may be configured to read out imagesusing rolling shutter or global shutter techniques. For example, toperform a rolling shutter readout using the pixel circuitry of FIG. 4A,a first reset may be performed to reset the sense node prior tointegration. Reset switch 404 may be opened to reset sense node 402 tothe second potential V_(BiasB). Closing reset switch 404 may initiate anintegration period, during which one or more measurements may be takento measure the potential of sense node 402 (which may vary as thephotosensitive layer absorbs light). A second reset may end integration.The period between the second reset and the first reset of a subsequentframe may depend on the frame readout rate.

Similarly, the pixel circuitry of FIG. 4A may adjust the first potentialV_(BiasT) to achieve a global shutter operation. In these instances thefirst potential V_(BiasT) may be driven at a first level duringintegration and at a second level outside of integration. The secondlevel of the first potential V_(BiasT) may be selected such that chargesgenerated in the photosensitive material are not collected by the pixelelectrode. A first reset may be used to reset the pixel electrode andsense node to the second potential V_(BiasB) at the start ofintegration. During integration (which may occur simultaneously acrossmultiple rows of the image sensor), the sense node potential may changebased on the amount of light absorbed by photosensitive layer 400. Afterintegration, the first potential V_(BiasT) may be returned to the secondlevel, and the charge on the sense node may be read out. A second resetmay again reset the sense node to the second potential, and a secondreading of the sense node may be read out. The multiple readings can beused, for example, in a correlated double sampling (CDS) operation.

Mitigation of Artifacts Due to Selective Readout

FIG. 5 is a schematic top view of an image sensor 500 with selectivereadout capability, in accordance with an embodiment of the invention.Image sensor 500 comprises a photosensitive medium, such as a quantumfilm, along with an array of pixel circuits on a semiconductorsubstrate, as illustrated in the preceding figures. These pixel circuitsdefine an array of pixels 502, which are arranged in a matrix of rowsand columns. In this and other figures, the rows extend horizontallyacross the array, while the columns extend vertically, but thisdesignation of “rows” and “columns” is arbitrary and is used solely forthe sake of simplicity and clarity of explanation. The pixel circuitsapply control signals to and read out photocharge from respective areasof the photosensitive medium. For this purpose, row logic 504 appliescontrol signals to the pixels in each row in order to reset and read outthe photocharge from the photosensitive medium as shown and describedabove.

Image sensor 500 is capable of selective readout, meaning that certainrows or windows comprising multiple rows are read out during a givenimage frame, while the remaining rows are cropped out or skipped over.For this purpose, control circuitry (for example, control circuitry 208in FIG. 2) programs row logic 504 to select a first set of rows of thepixel circuits, referred to as the active rows, to read out thephotocharge from the respective areas of the photosensitive medium in agiven image frame. Meanwhile, the accumulated photocharge is not readout during the given image frame by the pixel circuits in a second,remaining set of the rows, referred to as the skipped and cropped rows,which is disjoint from the first set. (The term “disjoint” is used inthis context in accordance with its conventional mathematical meaning,to indicate that the first and second sets have no rows in common.) Forexample, the first set of rows can define a cropping window, comprisingmultiple rows of the array belonging to the first set, while multipleconsecutive rows in the second set are outside the cropping window.Additionally or alternatively, at least some of the rows in the firstand second sets are sequentially interleaved in the matrix of pixels.

Although FIG. 5 shows alternating patterns of single read rows withsingle skipped rows (one such pattern beginning in row M and another inrow P), other alternation patterns are also possible. For example, in acolor image sensor, row logic 504 may be programmed to read alternatingpairs of rows (two read rows followed by two skipped rows, and soforth). As another example, rather than subsampling the rows in a ratioof 1:2, as shown in FIG. 5, the rows may be subsampled at lowerresolution, in which two, three or more rows are skipped for every rowthat is read out.

In image sensors that are known in the art, all the rows that are readout go through same reset process. If the sensor is programmed to besubsampled (i.e., to skip over rows at readout), however, or if it isprogrammed to read out only a cropped region, then the rows that areskipped or cropped do not go through this regular reset process. In suchcases, pixels of the photosensitive medium can subsequently exhibit adifference in their dark current or response, or both, depending onwhether they were previously read by an active row, including theregular reset, or were previously read by a row that was skipped overand therefore not reset. This difference can then undesirably appear inthe image when the sensor is switched from sub-sampling to regular mode,or from cropped to full-frame.

To avoid this sort of artifacts, row logic 504 in the present embodimentis programmed to control the pixel circuits so that in any given frame,all of the pixel circuits in both the active and skipped rows applyreset signals to their respective areas of the photosensitive medium.The pixel circuits in the active rows apply both these reset signals andsampling signals in order to read out the accumulated photocharge in thegiven frame. In the skipped rows, however, only the reset signals needbe applied.

More specifically, in an example embodiment, row logic 504 has thefollowing characteristics:

1. Row logic 504 resets every row (including the skipped and croppedrows) of the pixel array in the same way as the active rows.

2. This additional reset for the skipped and cropped rows can be veryfast, as the pixels in these rows are not sampled (i.e., these rows arenot read out), and therefore no settling of the signals from these rowsis required for accurate digitization. The use of a fast reset canreduce the impact on row time and thus increase the attainable framerate.

3. The reset of skipped rows in sub-sampling mode can be synchronizedwith the reset and readout of the adjacent active rows, which means thatunused rows are reset just before the adjacent row begins the readphase.

4. The top and bottom cropped rows can be reset at a different time(frame rate permitting) or in parallel with the readout of active rows.

5. As long as the number of active rows is larger than the number ofskipped and cropped rows, the skipped and cropped rows can be resetwithout substantial impact on the frame rate of image sensor 500.Otherwise, when there is a larger number of skipped and cropped rowsthan active rows, the need to reset the skipped and cropped rows maycause a reduction in the frame rate.

FIGS. 6 and 7 are timing diagrams, which schematically illustratepossible timing signals applied by row logic 504, including skip andcrop reset in two different modes of operation:

FIG. 6 shows a serial reset mode, in which a maximum of one row is resetat any given time. The skip and crop resets are added sequentially intime with application of the reset signals to the active rows, after theintegration reset signals. These sequential resets will increase the rowtime slightly (thus reducing frame rate). Since the pixels in theskipped and cropped rows are never read out, however, it is possible toperform a pixel reset in these rows of very short duration relative tothe reset in the active rows, as explained above. The impact on row timewill therefore be minimal as illustrated in FIG. 6.

FIG. 7 shows a parallel reset mode, in which some or all of the resetsare applied simultaneously, in both the active rows and the rows thatare skipped and cropped. In this configuration, the skip and crop resetsdo not add to the row time, so that there is no increase in the framerate. In this case, however, the reset circuitry in row logic 504 shouldhave enough driving capability to be able to reset a larger number ofrows simultaneously, for instance resetting five rows in the exampleshown in FIG. 7 in the same amount of time that it would have taken toreset just the three active rows.

Referring back to the example shown in FIG. 5, image sensor 500 isprogrammed in accordance with the following readout format:

a. Crop out rows from L to M−1, from O to P−1, and from Q to R.

b. Read rows M, M+2, . . . O−2, P, P+2, . . . Q−2. Rows M, M+2, . . .O−2 may be dark read rows, containing optically black pixels, which areread out in each frame and used for dark level calibration).

c. Skip rows M+1, M+3, . . . O−1 and P+1, . . . Q−1.

The above read, skip and crop regions are programmable and may bemodified by control circuitry 208 from frame to frame. The programmingmay be carried out, for example, by setting appropriate row addresspointers or flags, which instruct row logic 504 regarding which rows areto be read out, and which are to be skipped or cropped.

FIG. 8 is a table showing a flow of pointers used in controlling rowlogic 504, in accordance with an embodiment of the invention. Eachcolumn in the table show the address of the read, skip, and croppointers at a given row time. At each row time, the pointers areadvanced from one column in the table to the next. Thus, during thefirst row time, the read pointer address is M, the skip pointer addressis M+1, and the crop pointer address is L. At the next row time, thepointer addresses are advanced to M+2, M+3 and L+1, respectively, and soforth. Row logic 504 applies the row reset and readout signals inaccordance with the pointer locations at each row time. When a pointerhas finished going through all the rows of its type, it holds at acorresponding parking address until the next frame starts. The lastcolumn in the table represents the vertical blanking period.

The above schemes are shown and described by way of illustration, androw logic 504 may alternatively be designed and programmed in other waysthat still provide the desired reset of unread rows. The number ofcropped and skipped regions can be programmable to include dark, activeor any other type of rows.

Additionally or alternatively, the numbers of skip and crop pointers canbe greater than one of each and may be programmable, as well. Thus, forexample, if the number of read rows is less than the number of unusedcropped rows, and frame rate is a concern, then the number of croppointers could be increased so that all unused cropped rows will havebeen reset by the time the read pointer finishes traversing all the readrows. As another example, if the subsampling ratio is greater than 1:2,the number of skip pointers could be increased to match the additionalnumber of skipped rows that are to be reset for each active row. Thus,for 1:3 subsampling, there could be two skip pointers. When onlycropping or only subsampling (row skipping) is used, it is possible toenable only the crop pointer or only the skip pointer, but not both.

The direction of movement of the pointers could be from bottom to top(vertical flip mode), rather than top to bottom as shown in FIG. 8.Alternatively, the pointers could start from the top and bottom andconverge to the middle of the array.

As yet another option, the duration of the reset pulses can vary amongdifferent rows and regions. This variation can be used to compensate forresponse variations between regions, including those due to materialvariations or use of different types of optically-sensitive material onthe image sensor. For high dynamic range (HDR) image sensors, the pulsedurations can vary between the rows corresponding to differentintegration times in order to compensate for any difference in theresponse of these row types when switching from subsampled HDR tonon-HDR modes.

Mitigating Crosstalk Between Adjacent Pixels

In film-based image sensors, there is a stack of metal layers connectingthe photosensitive medium with the pixel circuit in each pixel, forexample as shown above in FIGS. 3B and 3C. These metal stacks can besensitive to electrical crosstalk between the metal stacks in adjacentpixels and between the metal stacks and pixel readout lines. Thiselectrical crosstalk is dependent on the size and geometry of the metallayers, as well as on light intensity. It can result in undesirablephenomena such as color casting, color hashing, and gain mismatchbetween different columns of the sensor array.

In some embodiments of the present invention, control circuitry in orassociated with the image sensor, such as control circuitry 208 shown inFIG. 2, applies a correction to each pixel based on the signal levels ofone or more neighboring pixels in order to cancel out the effect of thiscrosstalk. In other words, as the control circuitry receives signalsfrom the pixel circuits in the image sensor, corresponding to thephotocharge read out from respective areas of the photosensitive medium,and converts the signals to respective pixel output values, it correctsfor crosstalk of the photocharge from neighboring areas of thephotosensitive medium. This correction is typically applied digitally,row by row, after digitizing the signals from the image sensor, but itmay alternatively be applied, in either digital or analog circuits, atother points in the processing pipeline.

FIG. 9 is a schematic top view of a color image sensing array 900,illustrating the present method for correction of crosstalk, inaccordance with an embodiment of the invention. In this example, a colorfilter array overlies the photosensitive medium (such as a quantum film,as shown in the preceding figures) of array 900 and defines pixels 902of different colors in the corresponding areas of the photosensitivemedium. In the present embodiment, the control circuitry corrects therespective pixel output values for each given color for the crosstalkfrom the neighboring pixels of a different color.

Thus, in the pictured example:

-   -   Pixels in even rows of even columns are denoted Gb_(i).    -   Pixels in even rows of odd columns are denoted B_(i).    -   Pixels in odd rows of odd columns are denoted R_(i).    -   Pixels in odd rows of even columns are denoted Gr_(i).

-   Here i is an integer number. Furthermore, as shown in the figure:    -   B0, Gb0, R0 and Gr0 are the pixels to be corrected for        crosstalk.    -   Gb1, Gb2, B1, B2, R1, R2, Gr1 and Gr2 are the left and right        neighbors of the above pixels, i.e., the preceding and        succeeding pixels 902 in readout of the respective rows from        array 900.    -   B0′, Gb0′, R0′ and Gr0′ are the new pixel output values after        correction

The control circuitry corrects for crosstalk by reducing each of theoutput values from array 900 by an amount proportional to thephotocharge read out from either a preceding pixel, a succeeding pixel,or both, along a row of array 900. For this purpose, the controlcircuitry can store a table of weighting coefficients. The controlcircuitry then reduces the output value of each pixel by multiplying thesignals received from the preceding and/or succeeding pixel by theweighting coefficients to give a weighted correction, and subtractingthe respective weighted correction from each pixel signal. The weightingcoefficients may have fixed values, or they may alternatively vary as afunction of the signals received from the preceding and/or succeedingpixels in order to compensate for nonlinear effects.

Referring to FIG. 9 and the pixel definitions listed above, thecorrected pixel output values are computed as follows:

B0′=e·B0−b1·Gb1−b2·Gb2

Gb0′=e·Gb0−a1·B1−a2·B2

Gr0′=o·Gr0−d1·R1−d2·R2

R0′=o·R0−c1·Gr1−c2·Gr2

Typically, the coefficients a1, a2, b1, b2, c1, c2, d1 and d2 arefloating point numbers in the range [0,1], and o and e are floatingpoint numbers in range [1, +infinity] (although in practice, o and e aregenerally limited to the range [1,2]).

Alternatively, a one-sided crosstalk correction may be applied, in whicheach pixel is considered to be affected only by its predecessor orfollower. In this case, only the left neighbor may be used (i.e.,b2=a2=d2=c2=0) or only the right neighbor (i.e., b1=a1=d1=c1=0).Alternatively, the crosstalk correction may be pairwise, so that theleft pixel in each pixel pair is used to correct the right pixel, andthe right pixel of the pair is used for correcting the left pixel (inwhich case b1=a2=d1=c2=0).

Typically, the above corrections are implemented in hardware logic. Theone-sided approach (particularly using pairwise correction) hasadvantages of lower power consumption and gate count. The inventors havefound that the coefficients can be represented with sufficient precisionusing a two-bit value for the integer part and an eight-bit value forthe decimal part of the coefficients. To economize the logic stillfurther, the coefficients o and e can be fixed at the value 1, andinteger part of the remaining coefficients can be fixed at 0.

The above corrections work for both color and monochrome sensors, aswell as both visible and infrared sensors, though typically withdifferent values of the coefficients.

Nonlinear crosstalk can be handled using a piecewise-linear lookup tableto model the intensity-dependent coefficients. In this case, the controlcircuitry will read out and use the closest coefficient corresponding tosensed signal level of the neighboring pixel. Such a lookup table can beself-calibrating, with reprogramming of the coefficient values by thecontrol circuitry. Alternatively, the crosstalk coefficient fordifferent intensity settings can be preprogrammed in memory for eachimage sensor, for example using a one-time programmable (OTP) memory.

Mitigating Crosstalk Between Adjacent Rows

In film-based image sensors, such as those described above, there iscapacitive coupling between the sense nodes of adjacent rows of thepixel array. When operating in a rolling shutter mode, the pixelcircuits apply control signals to and read out photocharge fromrespective areas of the photosensitive medium (such as a quantum film)in a rolling sequence over the rows of the array. In other words, whilepixels in the same row have the same timing, the reset and readoutsignals applied to pixels in each successive, neighboring row areshifted by one row time (ΔT_(row)), typically on the order of 10 μs,relative to the preceding row.

Each pixel in a given row j integrates photocharge at a time that startswhen a pulsed integration reset signal (RST2) is applied to the sensenodes of the pixels in the row, and then reads out the integrated chargesubsequently upon application of a readout signal, which again resetsthe pixels (RST1). The integration reset pulse RST2 of the next row j+1is typically applied during the integration interval between RST2 andRST1 for row j. RST2 in row j+1 can cause a sharp jump in the voltage onthe sense node of the pixel in row j+1, by as much as several volts,which couples capacitively into the sense node of the neighboring pixelin row j during its integration interval. This crosstalk can causesignificant deviations in the readout voltage of the neighboring pixel,particularly when short integration times are used.

The present embodiment resolves this crosstalk problem by modifying therow control logic (for example, row circuitry 204 in FIG. 2 or row logic504 in FIG. 5) to enable application of an additional lock signal (RST3)in each image frame: Prior to the integration reset signal RST2, theRST3 lock signal is applied to gate 404 (FIGS. 4A/B) to set and hold thephotosensitive medium at the baseline voltage for at least one row time.(This lock signal is applied before or currently with the integrationreset signal in the preceding row of the array.) The reset signal RST2then follows the lock signal, and releases the photosensitive mediumfrom the baseline voltage, thereby initiating integration of thephotocharge by the pixel circuit. The readout signal RST1 follows thereset signal, and switches the photocharge that has been integrated outof the array.

FIG. 10 is a plot of voltage at the sense nodes (V_(SN)) of two pixelsin an image sensor, illustrating the use and effect of a reset locksignal in accordance with an embodiment of the invention. The voltage atthe sense node of a given pixel in one row is shown by an upper curve1002, while that of the neighboring pixel in the next row down in thearray is shown by a lower curve 1004. The image sensor is assumed to bean electron-accumulation sensor, so that the voltage on the sense nodesdrops following reset. The reset and readout signals (RST2 and RST1)occur in curve 1004 exactly one row time later than in curve 1002. Toillustrate the effect of the lock signal RST3, this lock signal is shownonly in curve 1004, although normally the lock signal could be appliedto all rows.

In the pictured example, the pixels circuits operate at a shortintegration time (i.e., the timespan between RST2 and RST1 is smallcompared to the time from RST1 to the next RST2), and the lower pixelaccumulates photocharge at a much faster rate than the upper pixel(e.g., due to a higher rate of incident photons from a bright object).Consequently, the lower pixel experiences a large voltage swing whenreset, resulting in a crosstalk artifact 1006 in curve 1002. If thisartifact were to occur during the integration time between RST2 and RST1in curve 1002, it would result in a substantial change in the signalread out from the upper pixel.

In the present embodiment, however, the lock signal RST3 is applied tothe lower pixel prior to the reset signal RST2 of the upper pixel, andthus prior to the beginning of the integration time of the upper pixel.The time span between the lock signal RST3 and the reset signal RST2applied to the lower pixel, ΔT, is sufficient to ensure that crosstalkartifact 1006 will occur prior to the beginning of the integration timeat the upper pixel, so that the crosstalk is essentially zeroed out byapplication of the reset signal RST2 to the upper pixel.

FIG. 11 is a plot illustrating the use and effect of the reset locksignal in a hole-accumulation image sensor, in accordance with anotherembodiment of the invention. The operation and effect of the reset locksignal RST3 is the hole-accumulation sensor is similar to that in theelectron-accumulation sensor of FIG. 10, except that the directions ofvoltage change are reversed. In FIG. 11, the voltage at the sense nodeof the current pixel is shown by an upper curve 1102, while that of theneighboring pixel in the next row down in the array is shown by a lowercurve 1104, with a crosstalk artifact 1106.

Various modes of implementation of the reset lock signal are possible.For example, a system of pointers could be used in controlling row logic504, in the manner described above with reference to FIG. 8. The rowtime for the image sensor will then comprise a read phase defined by acorresponding read pointer, one or more reset phases with correspondingreset pointers, and one or more additional reset lock pointers, whichwill hold each row in reset before the integration reset for that row,as explained above. The lock reset is disabled just before theintegration reset of the row. The number of row times over which aparticular row can be held in reset is programmable, so that lock-basedcrosstalk mitigation can extend over two or more adjacent rows. To avoidbright light lag, however, the voltage at the sense node should be heldat the baseline for no more than the minimum duration necessary, meaningthat the optimal number of row times during which pixels should belocked is two in most circumstances.

To prevent static bands in the image output by the image sensor, it isdesirable that the circuit activity be same for all rows. For thispurpose, the lock pointer can be run in the same way in the verticalblanking rows of the array as in the active rows. In the pointer-basedimplementation of the row logic that is described above, the lockpointer can run through the parking rows (typically including two ormore such rows) during vertical blanking.

The row logic in each row receives one lock pointer for each integrationreset pointer. Thus it is possible to define multiple integrationperiods for each row in a given frame. Each integration period isinitiated by a corresponding lock signal, followed by a reset signal,with timing selected so as to avoid crosstalk with the preceding row orrows in each one of the integration periods. The timing of lock andreset signals can also be adapted to configurations in which the pixelsof the image sensor are sub-sampled (skipping pixels or rows of pixels)or binned together.

In some embodiments, it can be advantageous to perform a so-called“hard/soft reset” operation, in which pixels are first flushed to avoltage significantly below supply (VDDA), after which the resettransistor is brought into the subthreshold regime in order to achievelower kTC noise. In this case, locked pixels undergo a sequence of“hard/soft” resets (one per row in parallel mode or three per row inserial modes), and it can be advantageous to first unlock the pixels andthen perform a regular pixel reset similar to the one performed by theread pointer.

Other embodiments are applicable to schemes of pixel noise reductionthat make use of a feedback circuit, which directly drives the pixelduring reset in order to suppress noise fluctuations. To facilitatethese types of noise cancellation techniques, only the current pixel isread out to the column bus (which is used by the feedback circuit as aninput), and the lock row operation is performed with the locked rowsdisconnected from the column bus (i.e., select transistor off, asillustrated in FIGS. 4A/B, for example).

Efficient Implementation of Optical Black Pixels

As noted earlier in reference to FIG. 2, image sensor 200 comprises anoptically black area, referred to as an obscured region 210, in whichpixels 212 do not receive light. In image sensors that are known in theart, an optically-opaque layer, such as an organic black layer, isdeposited over these pixels in order to block incident light. Controlcircuitry 208 uses the signals from these pixels in dark currentcompensation and correction of fixed pattern noise.

FIG. 12 is a schematic sectional view of a part of an image sensor 1200,showing a more efficient implementation of such an optically black area,in accordance with an embodiment of the invention. Image sensor 1200comprises a semiconductor substrate and an array of pixel circuits,arranged in a matrix on the semiconductor substrate and definingrespective pixels, as shown in the preceding figures. In FIG. 12, pixelcircuits 1202 and 1204 in metal layer Mx define a light sensing pixeland a black pixel, respectively. Pixel circuits 1202 and 1204 areconnected by vias 1210 to respective pixel electrodes 1208, formed inmetal layer My. Pixel electrodes 1208 contact a photosensitive film1206, such as a quantum film, formed over the pixel electrodes. A commonelectrode 1207, which is at least partially transparent, is formed overphotosensitive film 1206.

Control circuitry 1212 applies a bias to common electrode 1207 viaintervening contacts 1216 and an opaque metallization layer 1214, whichmakes ohmic contact with the common electrode. Opaque metallizationlayer 1214 is formed in metal layer Mz and extends over photosensitivefilm 1206 on the pixels of image sensor 1200 that are designated asblack pixels, such as the pixel defined by pixel circuit 1204 in FIG.12. Opaque metallization layer 1214 thus serves the dual purposes ofapplying the electrical bias to common electrode 1207 and opticallyblocking light from reaching the black pixels. This dual use of layer1214 reduces the number of process steps needed to produce image sensor1200. A buffer layer 1218 covers part of common electrode 1207 (as shownin the figure), or may extend over the entire surface of the commonelectrode.

Control circuitry 1212 receives signals from pixel circuits 1202, 1204,. . . , due to photocharge accumulated by pixel electrodes 1208 inresponse to application of the bias on common electrode 1207, andconverts the received signals to respective pixel output values. Thecontrol circuitry corrects the black level of the output values usingthe signals received from the black pixels, over which opaquemetallization layer 1214 is formed, as a reference. This same opaquemetallization layer, labeled Mz, may be further patterned to serve otherpurpose, such as creating thicker contact pads 1220 connecting tocontrol circuitry 1212.

Dual Image Sensor on a Common Substrate

In imaging applications targeted towards authentication, augmentedreality and virtual reality, it is often desirable to capture images ofthe same scene simultaneously in both the visible and infrared spectralregions. Certain image features for purposes of recognition andidentification are better captured in the infrared spectrum, and thesefeatures can then be combined with an image in the visible spectrum forhuman viewing and recording. When both visible and infrared images arecaptured with the same field of view and pixel count, the two images canalso be combined to yield depth information. In this way, featureextraction specific to certain wavelengths can be combined with depthand color information. Although imaging schemes that combine visible andinfrared sensing are known in the art, they tend to suffer frompractical limitations and high cost, which has limited the adoption ofsuch schemes in commercial applications.

Some embodiments of the present invention address the need for a dualvisible/infrared camera solution using the unique properties ofphotosensitive-film-based image sensors, such as quantum film sensors.In such solutions, a quantum film is deposited over a semiconductorwafer, such as a silicon wafer, with suitable circuits defining thepixels, made by a CMOS process, for example, before deposition of thefilm. Since the quantum film can be tuned to be sensitive to visible orinfrared spectra, two different types of quantum film can be patternedover two arrays of pixel circuits on the same wafer, with everythingbelow the quantum films being identical. Alternatively, the same type ofquantum film can be formed over both arrays of pixel circuits, with theaddition of suitable filter layers to select the wavelengths that willbe incident on each of the quantum films. In either case, two arrayswith different wavelength ranges, for example one sensitive to visiblelight and the other to infrared, can be manufactured next to each other,with the same pitch and circuit behavior, and with precisely-controlledspacing between the arrays.

FIG. 13 is a schematic, sectional illustration of a dual image sensorassembly 1300 of this sort, in accordance with an embodiment of theinvention. A semiconductor substrate, such as a silicon wafer 1302,comprises multiple sensing areas—including two such sensing areas 1306and 1308 in the present example. The sensing areas may correspondgeometrically to different, adjacent dies on wafer 1302, since they arepatterned with the same arrays of pixel circuits 1312, and they areseparated by a predefined distance corresponding to a “street” 1310 thatis normally left blank (unpatterned) between adjacent dies. In thepresent case, however, sensing areas 1306 and 1308 will not be dicedapart, but are rather kept together as parts of the same chip. Pixelcircuits 1312 define respective matrices of pixels in sensing areas 1306and 1308.

A film layer 1304 formed over wafer 1302 comprises photosensitive films1314 and 1316 over the arrays of pixel circuits 1312 in sensing areas1306 and 1308, respectively. A spacer 1318, of approximately the samewidth as street 1310, can be formed in layer 1304 between films 1314 and1316. In the pictured example, films 1314 and 1316 comprise different,respective materials, which are respectively sensitive to incidentradiation in different spectral bands, for example visible and infraredbands as marked in the figure. Films 1314 output photocharge to pixelcircuits 1312 in response to radiation incident in the respectivespectral bands, and sensing areas 1306 and 1308 thus output visible andinfrared image signals, which can be mutually registered in time andspace.

Alternatively, films 1314 and 1316 may comprise an identical film havinga spectral response extending over both the visible and infraredspectral bands. In this case, one or more optical filters with suitablepassbands may be overlaid or otherwise superposed in front of film layer1304 in order to differentiate the spectral responses of the films. (Anarrangement of this sort is described hereinbelow with reference to FIG.14.) Further alternatively or additionally, such optical filters may beused in combination with wavelength-selective quantum films.Furthermore, although the present examples relate specifically, for thesake of concreteness and clarity, to imaging assemblies that combinevisible and infrared sensing, the principles of these embodiments may beextended to any suitable combination of spectral bands in the visible,infrared and/or ultraviolet ranges, with two, three or more pixel arrayson the same chip, each with its own spectral band.

Further alternatively, films 1314 and 1316 and sensing areas 1306 and1308 may be configured to sense the same wavelength band, possibly withdifferent levels of sensitivity.

The spacing between sensing areas 1306 and 1308 can be set for thespecific requirements of the application in which assembly 1300 is to beused. For example, the spacing can be chosen to enable stereoscopicimaging by correlating the locations of objects appearing in the visibleand infrared images. Assuming that assembly 1300 is installed in aminiature camera module with a lens having a pupil diameter of 0.6 mmand an f number of 2, the approximate angular resolution of the modulewill be 0.1° at 940 nm. In order to match this angular resolution, thecorresponding parallax distance between sensing areas 1306 and 1308 forobjects located at 5 m, for example, is 9.1 mm.

Alternatively, the optics associated with assembly 1300 may imagedifferent, respective fields of view onto sensing areas 1306 and 1308.For example, the fields of view may overlap partially, with sensing area1306 configured for wide-angle imaging and sensing area 1308 fortelephoto operation.

Although pixel circuits 1312 in sensing areas 1306 and 1308 haveidentical geometries, they can be operated in different ways. Forexample, visible sensing area 1306 can operated in a rolling-shuttermode, while infrared sensing area 1308 operates in a global-shuttermode. This operational mode can be conveniently used in combination withstructured infrared light, which can be pulsed in synchronization withthe global-shutter timing in order to reduce power consumption of theassembly, as well as reducing the effect of ambient background on thestructure light image. The global-shutter infrared sensing area cansense the structured light pattern to provide depth information, whilethe visible sensing area provides two-dimensional image information.Alternatively, the infrared sensing area can operate in rolling-shuttermode while the visible sensing area operates in global-shutter mode,with or without structured light at an appropriate wavelength.

FIG. 14 is a schematic sectional illustration of an imaging module 1400with dual sensing areas 1306, 1308, in accordance with anotherembodiment of the invention. Sensing areas 1306 and 1308 are of similardesign and configuration to those described above. In the presentembodiment, however, the photosensitive films over the two sensing areascomprise identical films 1402, with the addition of suitable filters, asdescribed below. Arrays of microlenses 1404 are formed over films 1402in registration with the arrays of pixel circuits 1312. Imaging optics1406 focus respective images of a scene onto films 1402 over sensingareas 1306 and 1308. Alternatively, a single imaging lens (simple orcompound) can be used to focus images onto both sensing areas.

When identical films 1402 are formed over both sensing areas, opticalfilter 1408 with a visible passband can be inserted in the optical pathof sensing area 1306, with another optical filter 1410 with an infraredpassband inserted in the optical path of sensing area 1308.Alternatively or additionally, optical filter layers 1412 and 1414 maybe deposited over films 1402 on sensing areas 1306 and/or 1308. Forexample, filter layer 1412 may comprise a color filter array (CFA), suchas a Bayer filter array, in registration with the pixels in sensing area1306. In this case, filter 1408 may be configured to block infraredradiation, and filter layer 1414 may be replaced by a transparentspacer. Alternatively, module 1400 may comprise only filter layers 1412and 1414, without the addition of filters 1408 and 1410.

FIG. 15 is a schematic top view of an image sensor chip 1500 with dualimage sensing arrays 1502 and 1504, in accordance with a furtherembodiment of the invention. Sensing arrays 1502 and 1504 areconfigured, as described above, to sense light in two different spectralbands, such as visible and infrared bands, respectively. In thisembodiment, however, control circuitry is formed on the semiconductorsubstrate and communicates with the pixel circuits in both sensingarrays 1502 and 1504. Thus, in this case, there is no blank “street”between the arrays as in the preceding embodiments. Sharing of controlcircuitry in this manner can be useful in reducing chip area and cost,as well as facilitating close synchronization and integration betweenarrays 1502 and 1504.

Arrays 1502 and 1504 are row-aligned and thus share, in each row of thetwo arrays, common row decoder and driver circuits 1506, which are thecore of the timing engine of the readout circuitry. Column decoders andanalog/digital conversion circuits 1508, on the other hand, includeseparate components for the two sensing arrays. Logic and outputinterfaces 1510 perform higher-level on-chip processing and drive outputinterfaces to other devices, thus outputting image data from both ofarrays 1502 and 1504. Other circuitry 1512, also shared between arrays1502 and 1504, can include functions such as timing engines and controlof the operational mode of each individual array, for example enablingthe use of a rolling shutter in one array while running a global shutterin the other array.

In another embodiment (not shown in the figures), sensing arrays 1502and 1504 are column-aligned, rather than row-aligned. In this case, thesensing arrays share common column decoders and analog/digitalconversion circuits 1508 column by column, while row decoders and drivercircuits 1506 are coupled separately to the rows of each of the twoarrays. In other respects, resource sharing between arrays 1502 and 1504is similar to that shown in FIG. 15.

The configuration of FIG. 15 supports other sorts of resource sharingbetween arrays 1502 and 1504. For example, an opaque layer may be formedover the photosensitive film on one or more of the pixels in one of thearrays to create obscured region 210 (as shown in FIG. 2). Controlcircuitry on chip 1500, such as logic and output interfaces 1510, usesthe signals from the pixels in the obscured region in one of the arraysin correcting the black level of the pixel output values in both ofarrays 1502 and 1504.

As another example, row decoder and driver circuits 1506 can drivecommon electrodes over arrays 1502 and 1504 to apply a bias to thephotosensitive films on the arrays, thus enabling readout of signals dueto the accumulated photocharge. These common electrodes, correspondingto top electrode 306 (shown in FIG. 3), are at least partiallytransparent and are formed over the photosensitive films on the twoarrays. In some embodiments, row decoder and driver circuits 1506 biasthe common electrodes of both of arrays 1502 and 1504 at a commonpotential. Alternatively, row decoder and driver circuits 1506 bias thecommon electrodes at different, respective potentials.

Pixel Design for Phase Difference-Based Autofocus

Camera systems use autofocus (AF) in many applications to ensure thatrelevant portions of scenes, at varying distances from the camera, areacquired as in-focus image planes. Some autofocus systems use imageinformation output by the image sensor of the camera in estimating theoptimal distance of the image sensor from the camera lens. On-boardelectromechanical components then drive the lens position to the optimaldistance from the image sensor.

To improve autofocus performance, some cameras use dual-pixel autofocus,and particularly phase difference-based autofocus, based on signalsoutput by special pixels in the image sensing array that are dividedinto two sub-pixels. These special pixels can be created, for example,by fabricating a metal shield over certain pixels in such a way as toobscure one half of the sensing area of each such pixel.Phase-difference autofocus logic compares the outputs of the dividedsub-pixels in order to estimate of whether the image is in focus, andthus provides feedback in order to drive the lens to converge rapidly toa position at which the image is in focus.

Some embodiments of the present invention provide alternative types ofautofocus pixels that are appropriate particularly for image sensors inwhich a photosensitive medium, such as a quantum film, overlies an arrayof pixel circuits, which are arranged in a regular grid on asemiconductor substrate. The pixel circuits apply control signals to andread out photocharge from respective areas of the photosensitive medium,thus defining the pixels in the array. The term “regular grid” is usedto mean that the centers of the pixels in the array, as defined by thepixel circuits, are spaced apart by equal intervals in the horizontaland vertical directions. In typical arrays, the grid is arranged so thatthe pixels in the array are effectively square or rectangular.Alternatively, the pixels may be laid out on another sort of grid, suchas a hexagonal grid.

The autofocus pixels in the present embodiments differ from theremaining pixels in the array in that the pixel circuits of theautofocus pixels comprise conductive components, such as the pixelelectrodes or another metal layer below the pixel electrodes, that arespatially offset in different directions relative to the regular grid.The spatial offset may be expressed not only in terms of a shift in theconductive components, but also possibly in enlargement of theconductive components in the direction of the shift. In any case, sincethese same conductive components occur in all of the pixels in the array(though generally in regular, rather than offset, locations), theautofocus pixels can be fabricated as part of the array withoutrequiring any additional process steps.

Objective optics, such as lens 104 (FIG. 1), focus an image of an objectonto the photosensitive medium of the image sensor. Control circuitry,such as circuitry 208 (FIG. 2) or off-chip circuitry (not shown), readsout the photocharge from the pixel circuits and compares the photochargethat is output from pairs of autofocus pixels in which the conductivecomponents are offset in different directions. The control circuitryadjusts the focal setting of the objective optics based on thiscomparison.

FIG. 16 is a schematic sectional view of a part of an image sensor 1600,illustrating an example implementation of autofocus pixels, inaccordance with an embodiment of the invention. Image sensor 1600comprises a semiconductor substrate 1602, such as a silicon wafer, whichis overlaid by a photosensitive medium 1604, such as a quantum film. Anarray of pixel circuits 1606, such as the circuits in pixel circuitrylayer 302 (FIGS. 3A-3C), are formed in a regular grid on substrate 1602.Pixel circuits 1606 comprise pixel electrodes 1616, which are arrangedon a regular grid and are connected to the other circuit components byvias 1618, thus enabling the pixel circuits to apply control signals toand read out photocharge from respective areas of photosensitive medium1604. The pixels of image sensor 1600 in this example are overlaid by amosaic filter 1612 and respective microlenses 1614.

In contrast to the regular grid of imaging pixels defined by pixelcircuits 1606, pixel circuits 1608 and 1610 define a pair of autofocuspixels: In the autofocus pixel at the left in the figure, pixelelectrode 1616 is shifted to the right, while the pixel electrode in theautofocus pixel at the right in the figure is shifted to the left.Assuming the pixels to have a width and height of 1.1 μm, and the widthand height of electrodes 1616 to be 0.35 μm, shifting the electrodes inpixel circuits 1608 and 1610 of the autofocus pixels by 0.1 μm willincrease the sensitivity of these pixels to light coming in from oneside of the pixel by a factor of 1.5 to 2, relative to the other side.This difference in sensitivity is sufficient to enable the controlcircuitry to detect an imbalance upon comparing the outputs of the pairof autofocus pixels shown in FIG. 16, and thus to correct the focalsetting of the lens accordingly.

FIG. 17 is a schematic top view of a part of an image sensor 1700including autofocus pixels 1704, in accordance with another embodimentof the invention. As in the preceding embodiment, pixels 1702 define aregular grid, with pixel electrodes 1616 centered within these pixels.In autofocus pixels 1704, however, pixel electrodes 1616 are shifted inopposing directions relative to the grid of pixels 1702. The controlcircuitry can use the signals from autofocus pixels 1704 in the mannerdescribed above.

FIG. 18 is a schematic top view of a part of an image sensor 1800including autofocus pixels 1804, in accordance with yet anotherembodiment of the invention. In this case, again, pixels 1802 define aregular grid, with pixel electrodes 1616 centered within these pixels.In autofocus pixels 1804, however, pixel electrodes 1806 are bothshifted in opposing directions relative to the grid of pixels 1802 andare enlarged relative to the remaining pixel electrodes. Thisenlargement of the areas of pixel electrodes 1806, taken together withthe shift, can enhance the sensitivity of these pixels to light comingin from one side of the pixel by a factor of 2 to 3, relative to theother side.

FIG. 19 is a schematic sectional view of a part of an image sensor 1900,illustrating an alternative implementation of autofocus pixels, inaccordance with an embodiment of the invention. Most of the elements ofimage sensor 1900 are similar to those of image sensor 1600, asdescribed above with reference to FIG. 16, and are therefore labeledwith the same reference numbers. FIG. 19, however, shows an additionalmetal layer 1906 within the regular grid of pixel circuits 1902. Metallayer 1906 can not only serve as a conductive component within the pixelcircuits, but also reflects light that has passed through photosensitivemedium 1604 back into the photosensitive medium, thus enhancing thesensitivity of the corresponding pixel.

In the autofocus pixels defined by pixel circuits 1904, metal layer 1906is enlarged asymmetrically, thus creating an offset in these pixelsrelative to the remaining pixels in the array. The effect of this offsetis illustrated by the arrows in FIG. 19, which represent incoming raysof light that are incident on the autofocus pixels at relatively largeangles. Metal layer 1906 in each of the autofocus pixels reflects theangled rays on one side of the pixel back into medium 1604, but not onthe other side, thus enhancing the sensitivity of the correspondingpixels preferentially to incident light from different directions.

Although only a single pair of autofocus pixels is shown in each of thepreceding examples, in typical use multiple pixel pairs of this sort maybe formed, with opposing spatial phases in each pair due to offset ofthe pixel electrodes or other metal layer. The use of multiple autofocuspixel pairs ensures that there will be a sufficient number of autofocussamples in order to detect the best focal distance based on a selectedarea of the image (which can be selected by the user or automaticallyselected by the autofocus controller). It may also be desirable toinclude certain irregularities in the distribution of the autofocuspixels, in order to avoid aliasing effects that might otherwise beencountered.

When a given area of the image sensor is chosen for focusing, two setsof sub-images are analyzed by the autofocus controller: one sub-imagecomprising autofocus pixels that are preferentially sensitive to lightarriving from one direction (for example, from the left), and the othersub-image comprising pixels that are preferentially sensitive to lightfrom the other direction (for example, from the right). In anout-of-focus image region, the spatial frequency information from thescene will be mapped differently (with a different phase) onto the“left” sub-image and the “right” sub-image. The autofocus controlleruses this difference as a basis for determining the change required inlens-to-imager-distance. The controller considers the image in theregion of interest to be in focus when the image based on the “left”sub-image and the “right” sub-image are in phase, with maximalcorrelation between the sub-images.

To find the correct focal distance accurately over various regions ofthe image, it is desirable that the image sensor include a large numberof autofocus pixels. At the same time, it is desirable that these pixelscontinue to provide image information when they are not being used forautofocus measurements. One advantage of the present embodiments is thatthe autofocus pixels will continue to respond to incident lightintensity in a manner similar to the remaining pixels in the imagesensing array, though the pixel sensitivities may be slightly modifieddue to the differences in the locations and/or sizes of the metalcomponents of the pixel circuits. As the locations of the autofocuspixels in the array are known, their signal outputs can be corrected,for example by an on-line image signal processor (ISP), to correct forthe differences in sensitivity. Alternatively or additionally, when acolor mosaic film overlies the image sensors, the ISP may interpolatethe values of the autofocus pixels from other nearly pixels of the samecolor.

Although one of the examples presented above relates to pixels of acertain size, the principles of the present embodiments may be appliedin creating autofocus pixels of larger or smaller sizes, as well. Thecharacteristics and layout of the autofocus pixels may also be tailoredto fit the aberrations and chief ray angle characteristics of thespecific lens that is to be used in focus light onto the image sensor inquestion. In other words, the shifts and/or enlargement of theconductive components may be chosen in a way that is specific to thelocation of each pixel with respect to the center of the sensing array.

Image Sensors with Enhanced Acceptance of High Chief Ray Angles

In camera modules that incorporate an image sensor with a lens assembly,it is often desirable that the z-direction height of the total module(i.e., the dimension perpendicular to the image plane) be kept as low aspossible, while maintaining a wide field of view with the desired focallength and f-number. As the lens assembly is made shorter, the chief rayangle of incoming light at the edges of the image sensor becomes higher,i.e., farther from the normal to the image plane. This high chief rayangle leads to loss of sensitivity at the edges of the array.

To reduce these losses in conventional, silicon-based image sensors, themicrolenses and elements of the color filter array (CFA) may be shiftedinward, toward the center of the array, with the shift increasingradially with increasing distance from the center. As a result of theshift, more of the light at higher angles is collected at the edges ofthe array.

In image sensors that comprise a photosensitive medium, such as aquantum film, overlaid on an array of pixel circuits, the pixelelectrodes act as collectors of photocharge generated by photons thatare absorbed in the photosensitive medium. The present embodiments takeadvantage of this feature in increasing the sensitivity of the pixels tolight at high chief ray angles, by shifting the pixel electrodesradially outward relative to the center of the array. This feature canbe applied on its own or in combination with the microlens shiftdescribed above.

FIGS. 20A and 20B schematically illustrate a part of an image sensor2000 with enhanced acceptance of high chief ray angles, in accordancewith an embodiment of the invention. FIG. 20A is a sectional view, whileFIG. 20B is a top view. Image sensor 2000 comprises a photosensitivemedium 2004 overlaid on an array of pixel circuits, which are arrangedin a regular grid on a semiconductor substrate 2002, such as a siliconwafer, and define respective pixels 2006 of the image sensor. Each pixelhas a corresponding filter element 2008 in a color filter array and amicrolens 2010, which focuses incoming light onto the pixel.

Pixel electrodes 2012 read out photocharge from respective areas ofphotosensitive medium 2004 to the pixel circuits in each pixel 2006 ofthe array. To accommodate the increasing chief ray angles in theperipheral regions of the array, pixel electrodes 2012 are spatiallyoffset, relative to the regular grid of pixels, in respective directionsaway from a center of the array. In camera modules that includeobjective optics, such as lens system 104 (FIG. 1), which form an imageof an object on photosensitive medium 2004, the spatial offset of thepixel electrodes in the peripheral regions of the array can be set inaccordance with the chief ray angle of the light that is focused ontothe photosensitive medium by the objective optics, so as to minimize theloss of sensitivity in the peripheral regions.

The shifts of pixel electrodes 2012 may be applied incrementally, frompixel to pixel, as a function of radial distance and direction from thecenter of the array. Alternatively, for ease of design and production ofimage sensor 2000, the shifts may be applied in batches to differentgroups of the pixels, so that the same shift of the pixel electrodes isapplied in all the pixels in each group.

FIG. 21 is a schematic top view of a part of an image sensor 2100 withenhanced acceptance of high chief ray angles, in accordance with anotherembodiment of the invention. In this case, pixel electrodes 2112 inpixels 2106 in the peripheral regions of the pixel array of sensor 2100are enlarged in the appropriate radial directions relative to the pixelelectrodes in the center of the array. This enlargement is also usefulin offsetting the loss of sensitivity of the peripheral regions of imagesensor 2100 due to high chief ray angles. It may be appliedindependently of or in conjunction with the shift described above.

FIG. 22 is a schematic sectional view of a part of an image sensor 2200with enhanced acceptance of high chief ray angles, in accordance withyet another embodiment of the invention. As in the embodiment of FIGS.20A/B, image sensor 2200 comprises a photosensitive medium 2204 overlaidon an array of pixel circuits, which define respective pixels 2006 ofthe image sensor. Each pixel has a corresponding filter element 2208 anda microlens 2210, which focuses incoming light onto the pixel. In thiscase, however, microlenses 2210 are spatially offset, relative to theregular grid, in respective directions toward the center of the pixelarray, while pixel electrodes 2212 are shifted away from the center.This arrangement enhances the ability of the pixel electrodes to capturephotocharge due to light that is incident at high angles, such as a ray2214 shown in FIG. 22.

The inventors have found this latter arrangement, combining offset ofthe microlenses in one direction and the pixel electrodes in the other,to be particularly effective in achieving acceptable image sensorsensitivity at larger chief ray angles than can be accommodated usingonly shift of the microlenses. As demonstrated by specific examplesdescribed in the above-mentioned U.S. Provisional Patent Application62/411,522, combinations of microlens offset with electrode offset canbe used in cameras with both very low Z-direction height and large pixelarray dimensions, to accommodate chief range angles at the edges of theray in the range of 40° and even higher.

Although the embodiments described above relate mainly to film-basedimage sensors, the principles of these embodiments may similarly beapplied, mutatis mutandis, in image sensors of other types, using othersorts of photosensitive media. Furthermore, although various features ofimage sensors are described separately above, some or all of thesefeatures may be implemented together in a single image sensing device.

It will thus be appreciated that the embodiments described above arecited by way of example, and that the present invention is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present invention includes both combinations andsubcombinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art.

1. Imaging apparatus, comprising: a photosensitive medium; an array ofpixel circuits, which are arranged in a regular grid on a semiconductorsubstrate and define respective pixels of the apparatus; and pixelelectrodes connected respectively to the pixel circuits in the array andcoupled to read out photocharge from respective areas of thephotosensitive medium to the pixel circuits, wherein the pixelelectrodes in a peripheral region of the array are spatially offset,relative to the regular grid, in respective directions away from acenter of the array.
 2. The apparatus according to claim 1, wherein thephotosensitive medium comprises a quantum film.
 3. The apparatusaccording to claim 1, wherein the pixel electrodes in the peripheralregion of the array are enlarged in the respective directions relativeto the pixel electrodes in the center of the array.
 4. The apparatusaccording to claim 1, and comprising objective optics, which areconfigured to form an image of an object on the photosensitive medium,wherein the pixel electrodes in the peripheral region are spatiallyoffset by a displacement determined by a chief ray angle of theobjective optics.
 5. The apparatus according to claim 1, and comprisingmicrolenses formed over the photosensitive medium, wherein themicrolenses associated with the pixels in the peripheral region of thearray are spatially offset, relative to the regular grid, in respectivedirections toward the center of the array.
 6. A method for producing animage sensor, the method comprising: forming an array of pixel circuitsin a regular grid on a semiconductor substrate, thereby definingrespective pixels of the image sensor; connecting pixel electrodesrespectively to the pixel circuits in the array, wherein the pixelelectrodes in a peripheral region of the array are spatially offset,relative to the regular grid, in respective directions away from acenter of the array; and coupling the pixel electrodes to read outphotocharge from respective areas of the photosensitive medium to thepixel circuits.
 7. The method according to claim 6, wherein thephotosensitive medium comprises a quantum film.
 8. The method accordingto claim 6, wherein the pixel electrodes in the peripheral region of thearray are enlarged in the respective directions relative to the pixelelectrodes in the center of the array.
 9. The method according to claim6, and comprising arranging objective optics to form an image of anobject on the photosensitive medium, wherein the pixel electrodes in theperipheral region are spatially offset by a displacement determined by achief ray angle of the objective optics.
 10. The method according toclaim 6, and comprising forming microlenses over the photosensitivemedium, wherein the microlenses associated with the pixels in theperipheral region of the array are spatially offset, relative to theregular grid, in respective directions toward the center of the array.